The invention relates to a process of making useful shapes by joining of tungsten alloys. The quest for a high-density material having a unique combination of high strength and ductility, high modulus, and good corrosion resistance, resulted in the development of a class of alloy known as “tungsten heavy alloys.” Though there are several other high-density metals, none of them can rival tungsten in terms of high density and affordability. Some of the comparable high-density materials are either noble metals and are either extremely expensive or are toxic to the environment (gold, platinum, osmium, uranium, rhenium). Thus, tungsten is usually the material of choice for applications that require high density, such as kinetic energy penetrators, radiation shields, counterweights, etc. The solution to the problem of producing a high-density alloy from tungsten at a reasonable cost was first suggested to Smithells by Sir John Mclennan [1,2]. The sintering of pure tungsten, however, would require very high temperatures that would make the process very expensive. The problem was solved by mixing tungsten powder with a small proportion of a lower melting metal to form a liquid phase when heated to a moderate temperature. The metal additive would have to wet the tungsten particles, which would then result in shrinkage and attainment of near full density. Copper and nickel were chosen as the early additives (90W-5Ni-5Cu), and then sintering it in a hydrogen atmosphere at 1450° C. resulted in the desired material. The density of the finished piece was between 16.3 to 16.5 g/cc. The new alloy, therefore, had a density that was approximately 50% greater than that of lead and tensile strengths around 40 t/in2 and a tensile elongation of 4%.
There was soon an explosive growth in the research and development activities with tungsten heavy alloy systems. Once it was realized that this alloy had tremendous potential in defense related applications due to its high density and relatively low cost, a great deal of government funding spurred the continued processing improvements with this alloy system. It soon became clear that the properties of these alloys were greatly affected by numerous factors, especially the processing. Another important development was the unification of the alloy systems with W—Ni—Fe becoming the most popular tungsten heavy alloy with the nickel to iron ratio of 7:3 or 8:2 exhibiting optimum properties.
Typically, tungsten heavy alloys are two phase composites which normally consist of almost pure body centered cubic (bcc) tungsten grains embedded in a comparatively soft and ductile face centered cubic (fcc) matrix alloy. In conventional heavy alloys the tungsten content varies from 90 to 98 weight percent. Commonly the remaining alloy constituent usually contains nickel, iron, cobalt, and copper (usually a minimum of two elements are used, e.g. Fe:Ni, or Ni:Cu). The most popular additive being the nickel and iron in the ratio of 7Ni:3Fe or 8Ni:2Fe (weight ratio). The conventional means of processing the tungsten heavy alloys include the mixing of the desired amount of the elemental powders, cold pressing, followed by liquid phase sintering to almost full density. During liquid phase sintering, the matrix alloy melts and takes into solution some tungsten, resulting in a microstructure in which relatively large tungsten grains (20 to 60 11μ) are uniformly dispersed in a matrix alloy. This alloy usually is comprised of nickel and iron that has taken into solution some tungsten. Currently conventional tungsten heavy alloys exhibit a property combination that is unique. Properly processed material exhibits a combination of high density greater than 17 g/cc for a typical 90 weight percent tungsten containing alloy, high strength (often as high as 800 to 1000 MPa), high ductility ranging from 10 to 30%, good corrosion resistance, high radiation absorption capability, and reasonably high toughness. This unique combination of properties has made this alloy a candidate for both defense and civilian applications. Some of its applications include radiation shields, counter weights, kinetic energy penetrators, vibration dampening devices, several medical devices for containment of radioactive isotopes, heavy duty electrical contact materials, balancing crankshafts for internal combustion engines used in racing motor cars, gyroscopes, and targeted weights in golf club heads and putters.
Though the early work on this alloy system dates back to the 1930's, the tremendous interest in this alloy system was precipitated due to the possible use of this alloy in defense related applications. During the last quarter of this century, two distinct trends emerged, which had a major impact on the processing strategies adapted for producing tungsten heavy alloy parts. Tungsten heavy alloys were in direct competition with depleted uranium for use as kinetic energy penetrators. Work at the Argonne Research Laboratory showed that the superior properties of depleted uranium were a result of its ability to localize shear during ballistic penetration events. Thus, it was argued that if localized shear can be imparted to tungsten heavy alloys, these alloys would exhibit penetration performance that would match that of depleted uranium, an environmentally sensitive material. A tremendous spurt of research ensued in finding a heavy alloy that would be prone to adiabatic shear. The search for this ideal heavy alloy composition and processing combination resulted in several interesting processing strategies which includes processes such as solid state sintering, powder extrusion, coating of tungsten powders with the matrix materials, and the use of a tungsten heavy alloy core coated with an adiabatic shear prone alloy. The other important processing strategy was born out of the need for producing complex shaped tungsten heavy alloy parts. The commercialization of the technique of powder injection molding resulted in tremendous interest in the processing of these alloys into near net shapes. To address the problem of high strength heavy alloys without subjecting the material to a thermo-mechanical processing step, new alloys were developed. In these new heavy alloys, a part of the tungsten weight fraction was replaced by other refractory metals such as molybdenum, or rhenium which increased the strength of the alloys without the thermo-mechanical processing step.
Tungsten heavy alloys have some unique applications that require fabrication of very large (hundreds of kilogram) shapes such as hollow conical, hollow cylindrical, or even a one face open box-like structure. Processing of such shapes as one piece is usually very difficult and sometimes impossible. The problem stems from the basic manner by which the excellent properties of these alloys are achieved, which is by liquid phase sintering. During liquid phase sintering, a part of the structure becomes a liquid (volume being dependent on the composition and the processing conditions). If the structure has a large mass on top, it results in gravity induced slumping leading to distortion at the bottom of the structure, often termed as “elephant foot syndrome.”
The problem stems from the separation of the solid and liquid due to gravity somewhat like the segregation of water and sand particles. This gravity induced slumping has been the subject of several interesting research papers that also includes microgravity sintering. This behavior of liquid phase sintered materials creates a great deal of difficulty in the production of large liquid phase sintered tungsten heavy alloy structures in the normal gravity bound environment. Thus, to manufacture very large tungsten heavy alloy parts in a cost effective manner (without the aid of microgravity), the only solution seems to be the joining by high temperature diffusion bonding of smaller tungsten heavy alloy pieces to build up a very large component.
Some of the well known joining techniques that have been outlined in the literature include mechanical fastening, welding, brazing, adhesive bonding, reaction bonding, and soldering. These processes and some of their associated problems are extensively described in the open literature. In the case of tungsten heavy alloy joining, the need for attaining the parent material properties as well producing structures having a large mass (hundreds of kilograms) without significant slumping makes the technical issues far more challenging than normal joining related problems. The use of pressure assistance, which aids in the creation of a good joint, may not be a problem in the case of joining large pieces of tungsten heavy alloys, as the weight of the top heavy alloy piece will often be quite large and will provide significant pressure at the joint interface. Thus, the primary need is to provide at the interface certain material(s) that will provide a rapid diffusion path for tungsten/nickel/iron. The short circuit diffusion path could be aided with the formation of a thin layer of liquid at the interface. The key to the success was the choice of a temperature high enough to allow significant diffusion across the joint interface but definitely lower than the temperature that would cause melting in the parent tungsten heavy alloy structure.
The manufacturing of very large tungsten alloy components made by liquid phase sintering as one single piece is not technically feasible. Thus, joining of relatively smaller heavy alloy pieces to form a very large part is an attractive manufacturing route. In this latter method the smaller heavy alloy pieces can be liquid phase sintered because the mass is not too great to cause slumping, and then the subsequent joining has to be carried out at a temperature that does not exceed the solidus temperature of the heavy alloy pieces that are to be joined. This ensures the avoidance of the liquid phase and the concomitant slumping effect. The material that is used to join the pieces of liquid phase sintered heavy alloys can, however, be a liquid or remain solid at the joining temperatures.
One of the main criteria for the joining material is that it should have good solubility for tungsten and forms a strong interface so as to withstand launching. The U.S. Department of Defense has a need for the fabrication of such large tungsten alloy penetrators suitable for destroying buried concrete bunkers and deep down trenches. Currently the technology to manufacture such large penetrators in an economical manner is not available. This invention discloses the feasibility for the fabrication of these large tungsten alloy parts by joining smaller liquid phase sintered tungsten heavy alloy pieces.
This joining can be accomplished by several means. One of the techniques is to have a foil made of the desired composition. The choice of the materials has to be based on fundamental metallurgical concepts. The materials chosen have to impart good interfacial strength as well as have solubility for the tungsten to allow rapid diffusion of tungsten across the interface. The joining materials can have joint thickness typically below 100 μm. It is usually good to have a material with a melting point that is at least 100° C. lower than the parent heavy alloy pieces. Thus, this joining alloy when heated to a temperature above its melting point but below the melting point of the parent heavy alloy pieces being joined will form a liquid phase at the interface only without any melting of the remaining structure.
Other methods of joining include sputtering thin layers of a special alloy on the flat faces of a heavy alloy cylinder that needs to be joined, using sequential depositions of elements such as Cu and Ni by an electrolytic process or electrolysis process. Another method used involves the thermal evaporation of the desired composition on the flat surface of the heavy alloy cylinders to be joined. Investigation indicates the feasibility of producing large structures of tungsten heavy alloys using the technique of joining of smaller pieces together by high temperature diffusion bonding using foils, sputtering, and thermal evaporation. The uses of these structures can be used in radiation shields, smaller joint structures for the oil and gas industry, and of course in large penetrators.